Cryptochromes are blue light-absorbing photoreceptors found in plants and animals with many important signalling functions. These include control of plant growth, development, and the entrainment of the circadian clock. Plant cryptochromes have recently been implicated in adaptations to temperature variation, including temperature compensation of the circadian clock. However, the effect of temperature directly on the photochemical properties of the cryptochrome photoreceptor remains unknown. Here we show that the response to light of purified Arabidopsis Cry1 and Cry2 proteins was significantly altered by temperature. Spectral analysis at 15 C showed a pronounced decrease in flavin reoxidation rates from the biologically active, light-induced (FADH ) signalling state of cryptochrome to the inactive (FADox) resting redox state as compared to ambient (25 C) temperature. This result